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Because the parts of a mixture are not chemically combined, they can be pulled apart by physical methods — no chemical reaction is needed. The skill is to pick a method that takes advantage of a difference in physical properties between the components: a difference in solubility, in particle size, or in boiling point. This lesson, part of Topic C2 of your OCR Gateway Combined Science course, works through the four core separation techniques — filtration, crystallisation, simple distillation and fractional distillation — and shows how to choose the right one and combine them to purify a real mixture such as rock salt.
By the end of this lesson you should be able to describe filtration, crystallisation, simple distillation and fractional distillation, explain which physical property each one relies on, choose the right technique for a given mixture, and plan a multi-step separation.
This lesson develops AO1 (recalling how each separation technique works and the practical methods) and AO2 (applying the right technique to the physical property that differs), reaching AO3 when you plan and sequence a multi-step separation.
Filtration separates an insoluble solid from a liquid (or from a solution). The mixture is poured through filter paper in a funnel: the liquid passes through the tiny holes, but the solid particles are too big and stay behind.
For example, a mixture of sand and water can be filtered: the sand is the residue (it cannot fit through the paper) and the water is the filtrate. Filtration works because the two components differ in particle size — but only if the solid is insoluble. A dissolved solid passes straight through with the liquid, because dissolved particles are far too small to be caught, so filtration cannot remove salt from salt water.
Exam Tip: Filtration separates an insoluble solid from a liquid. Learn that the residue is the solid left in the paper and the filtrate is the liquid that passes through. If the solid is dissolved, filtration will not work — it goes through with the filtrate.
Crystallisation separates a soluble solid from the solution it is dissolved in — for example, getting copper sulfate crystals back from copper sulfate solution. It works by evaporating the solvent so that the dissolved solid is left behind as crystals.
Method:
Cooling and evaporating slowly gives larger, well-formed crystals, because the particles have time to arrange themselves into a regular pattern. You must not evaporate a hydrated salt to dryness with strong heating, because that can drive off the water of crystallisation (or even decompose the salt) and spoil the crystals.
Exam Tip: Crystallisation recovers a soluble solid from its solution. The marks are: evaporate some (not all) of the solvent, then cool to crystallise, then filter and dry. Slow cooling and evaporation give bigger crystals.
Simple distillation separates a liquid (the solvent) from a solution — for example, pure water from salt water. It relies on the difference in boiling point between the liquid you want and the dissolved solid, which has a much higher boiling point and will not boil off.
The solution is heated until the solvent boils and turns to vapour. The vapour passes into a condenser — a tube surrounded by a cold-water jacket — where it cools and condenses back into a liquid. This pure liquid, the distillate, is collected, while the dissolved solid stays behind in the flask.
Simple distillation is fine when there is a big difference in boiling point — such as that between a liquid solvent and a dissolved solid. It is not suitable for separating two liquids whose boiling points are close, because both would boil off together; for that you need fractional distillation.
Exam Tip: Distillation does not make a new substance — it is a physical change. The water you collect was always water; it has just been boiled off and condensed away from the salt. State that distillation relies on a difference in boiling point. This is a common misconception to avoid: the collected liquid is not "new".
Fractional distillation separates a mixture of two or more miscible liquids (liquids that mix completely) that have different boiling points — for example, ethanol from water, the different fuels in crude oil, or the gases in liquefied air.
The extra piece of apparatus is a fractionating column — a tall column, packed or filled with glass beads, fitted above the flask. It is hottest at the bottom and gets cooler towards the top. As the mixed vapour rises, it repeatedly condenses and re-evaporates on the packing, which separates the liquids by their boiling points:
For an ethanol–water mixture, ethanol (boiling point 78°C) distils over before water (100°C). You watch the thermometer: the temperature holds steady at the boiling point of whichever liquid is distilling, then rises to the next plateau as that liquid runs out and the next one begins.
| Technique | Separates | Physical property used |
|---|---|---|
| Filtration | Insoluble solid from a liquid | Particle size (solubility) |
| Crystallisation | Soluble solid from its solution | Solubility (solid stays when solvent evaporates) |
| Simple distillation | A liquid (solvent) from a solution | Boiling-point difference (solid vs liquid) |
| Fractional distillation | Two or more miscible liquids | Different boiling points of the liquids |
Exam Tip: Use fractional (not simple) distillation to separate liquids with similar boiling points. The key feature is the fractionating column (cooler at the top), and the liquid with the lowest boiling point comes off first.
When a question asks you to separate a mixture, work out what the components are and what differs between them. This decision flow captures the logic.
flowchart TD
A["What is in the mixture?"] --> B{"Insoluble solid<br/>in a liquid?"}
B -->|Yes| C["Filtration"]
B -->|No| D{"Soluble solid<br/>dissolved in a liquid?"}
D -->|"Want the solid"| E["Crystallisation"]
D -->|"Want the liquid"| F["Simple distillation"]
D -->|No| G{"Two or more<br/>liquids mixed?"}
G -->|"Similar boiling points"| H["Fractional distillation"]
Rock salt is a mixture of salt (soluble) and sand (insoluble). Describe how to obtain pure, dry samples of both the salt and the sand from rock salt.
Step 1 — add water and stir. The salt dissolves; the sand does not (it is insoluble). You now have salty water with sand in it.
Step 2 — filter. The sand is trapped as the residue in the filter paper; the salt solution (salty water) passes through as the filtrate. Wash the sand with a little water and leave it to dry — that gives the pure, dry sand.
Step 3 — take the salt solution (filtrate) and crystallise it: heat it to evaporate some of the water until crystals begin to form, then leave it to cool so the salt crystallises out.
Step 4 — filter off the salt crystals and dry them. That gives the pure, dry salt.
Answer: dissolve → filter (sand = residue, dry it) → crystallise the filtrate → filter and dry the crystals (salt). One mixture, two pure products, using only physical methods.
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